Spanners and Sockets, Equivalent Sizes

The following table covers common (and not-so-common) nut and bolt sizes, in AF, Metric, BSW and BSF. Below the table is an explanation of why the different standards exist, and how they are often compatible. All material provided by Pete Kay (www.eraturbo.com)

Across flats		Designated socket or spanner
Inches	Millimetres
0.2500	6.35	1/4” AF.
0.2756	7.00	7mm
0.3125	7.94	5/16” AF
0.3149	8.00	8mm
0.3438	8.74	11/32” AF and 1/8” BSW
0.3543	9.00	9mm
0.3750	9.52	3/8” AF
0.3937	10.00	10mm
0.4330	11.00	11mm
0.4375	11.11	7/16” AF
0.4450	11.30	3/16” BSW and 1/4” BSF
0.4724	12.00	12mm
0.5000	12.70	1/2” AF
0.5118	13.00	13mm
0.5250	13.335	1/4” BSW and 5/16” BSF
0.5512	14.00	14mm
0.5625	14.288	9/16” AF
0.5906	15.00	15mm
0.6000	15.24	5/16” BSW and 3/8” BSF
0.6250	15.875	5/8” AF
0.6299	16.00	16mm
0.6693	17.00	17mm
0.6875	17.463	11/16” AF
0.7087	18.00	18mm
0.7100	18.034	3/8” BSW and 7/16” BSF
0.7480	19.00	19mm
0.7500	19.05	3/4” AF
0.7874	20.00	20mm
0.8125	20.637	13/16” AF
0.8200	20.828	7/16” BSW and 1/2” BSF
0.8268	21.00	21mm
0.8661	22.00	22mm
0.8750	22.25	7/8” AF
0.9055	23.00	23mm
0.9200	23.368	1/2” BSW and 9/16” BSF
0.9375	23.825	15/16” AF
0.9449	24.00	24mm
0.9843	25.00	25mm
1.0100	25.654	9/16” BSW and 5/8” BSF
1.0236	26.00	26mm
1.0625	26.988	1.1/16”AF
1.0630	27.00	27mm
1.1000	27.94	5/8” BSW and 11/16” BSF
1.1024	28.00	28mm
1.1250	28.6	1.1/8" AF
1.1417	29.00	29mm
1.1811	30.00	30mm
1.2000	30.48	11/16” BSW and 3/4” BSF
1.2205	31.00	31mm
1.2500	31.75	1.1/4” AF
1.2598	32.00	32mm
1.2992	33.00	33mm
1.3000	33.02	3/4” BSW and 7/8” BSF
1.3125	33.338	1.5/16” AF
1.3386	34.00	34mm
1.3780	35.00	35mm
1.3900	35.306	13/16” BSW and 15/16” BSF
1.4173	36.00	36mm
1.4375	36.512	1.7/16” AF
1.4567	37.00	37mm
1.4800	37.952	7/8” BSW and 1” BSF
1.4961	38.00	38mm
1.5000	38.10	1.1/2” AF
1.5354	39.00	39mm
1.5748	40.00	40mm
1.6142	41.00	41mm
1.6250	41.275	1.5/8” AF
1.6535	42.00	42mm
1.670	42.418	1” BSW and 1.1/8” BSF
1.6875	42.863	1.11/16” AF
1.6929	43.00	43mm
1.7323	44.00	44mm
1.7717	45.00	45mm
1.8110	46.00	46mm
1.8125	46.037	1.13/16” AF
1.8504	47.00	47mm
1.8600	47.244	1.1/8” BSW and 1.1/4” BSF
1.8750	47.625	1.7/8” AF
1.8898	48.00	48mm
1.9291	49.00	49mm
1.9685	50.00	50mm

So why 4 measuring standards?

When the Mini was designed in the in the Nineteen Fifties, (when I was in my late teens) Metric threads, and spanners marked with their Across Flat (AF) size in millimetres were very rare, except in continental Europe.

Nearly all British cars (except Ford) used either British Standard Whitworth (BSW) or British Standard Fine (BSF) threads for any screw application. The former was a course thread, used for larger screws, which varied in pitch according to its diameter, and the finer threaded BSF was used for short screws, which was more applicable to motor vehicles. Both these screws have an angle of 55deg between threads, with the thread width, and hence pitch, varying in relation to diameter.

Because Ford cars were designed to American standards, they used American screw threads, which were termed National Coarse (NC) and National Fine (NF). They were similar to the English versions, but had a 66deg thread angle. Due to problems in WW2 with thread sizes, the sizes of these American threads were 'unified' between Britain, Canada and America and, from about 1960 they became UNC and UNF.

Now to the point of the story. The British retained spanners marked with BSW and BSF sizes, which related to the thread diameter, not the across flats (AF) size used commonly but incorrectly by the Americans for their UNF and UNC screws. In fact, each spanner was marked with both Whitworth and BSF sizes, which are directly related but not the same.

For any given imperial size, up to 7/8 inch BSF, the Whitworth equivalent is always 1/16th of an inch smaller than the equivalent BSF. e.g. a 1/2 inch BSF screw has the same across flats size as a 7/16inch Whitworth screw (or nut). Hence the dual marking of the spanner. For larger sizes, the difference is 1/8 inch.

Unfortunately, this does not indicate the across flats (AF) size of the screw or nut. The Americans, however did standardise the Unified screws to have a specific relationship between the screw diameter and head size across flats. This relationship is the same as that used by the English for the BSF sizes. For example, a bolt with a thread diameter of 1 inch, in either Unified or BSF has a head measurement of 1.5inch AF. Similarly a 3/4 inch diameter thread has a head which is 1.1/8 inch across flats.

Be Warned. There are not many precise equivalents from BSW/BSF to imperial AF or Metric sizes. Most nearly fit!